1. Field of the Invention
The present invention relates to a sealing device for steam turbines.
2. Description of the Related Art
Power-generating plants that generate electric power by rotating steam turbines with the steam generated by boilers or the like are commonly equipped with, in order from the upstream side of the flow of the steam, high-pressure turbines, intermediate-pressure turbines, and low-pressure turbines, as the steam turbines. The steam that has been used to rotate a low-pressure turbine is introduced into a condenser via an exhaust hood and then condensed by the condenser to become feedwater. The feedwater is recycled into the steam generator.
In the steam turbines constituting such a power plant, stationary blades fixed to the inside of a casing are arranged between the moving blades that rotate integrally with a rotor, and stages including the stationary blades and the moving blades are formed.
The steam that has been introduced into the casing of the steam turbines flows through the inside of the casing and passes through alternate spaces between the stationary blades and the moving blades fixed to the rotor rotatably supported by the casing. The steam expands during the alternate movements, and rotates the rotor. The steam passes through the moving blades equipped at the most downstream position on the rotor, that is, the moving blades of a final stage, and is released from the casing.
More specifically, the stationary blades are most often fixed to the inside of a diaphragm mounted internally to the casing, and the moving blades are fixedly mounted on an outer surface of the rotor to rotate integrally with the rotor. The casing and the diaphragm are most commonly divided into two sections, upper and lower, from a viewpoint of ease in maintenance.
In such a steam turbine, since steam rotates the rotor by rotating the moving blades, it is required that sealing performance between a fixed region and rotating region of the turbine be improved to use the steam more efficiently with minimum steam leakage from such clearances as between the rotor and the diaphragm containing the stationary blades and retained in the casing, and between a distal end of each moving blade and the casing.
If the clearances between the rotating region and the fixed region are dimensionally reduced for improved sealing performance, however, damage to sealing fins due to contact between the sealing fins and the rotor is more likely to occur. If the damage actually occurs, this in turn causes a problem of increased steam leakage.
Fin contact between the rotating region and the fixed region is considered to be caused by thermal deformation of the casing, the diaphragm, and/or the like. It is well known that a difference in thermally insulated state between the upper and lower sections of the casing causes a difference in surface temperature between both, thus elongating the casing more significantly on the upper side than on the lower side. That is to say, the casing becomes bulged or humped upward to suffer thermal bending deformation, which moves the casing (fixed region) upward relative to the rotor (rotating region) and causes rotor-fin contact.
If a difference in temperature occurs in a radial direction of the diaphragm, the diaphragm becomes hotter on its inner surface than on its outer surface, with the result that the inner surface of the diaphragm elongates relative to the outer surface. This thermal elongation causes the diaphragm to suffer bending deformation, and the inner surface of the diaphragm to move upward relative to an end portion of its outer-surface dividing face. The lower diaphragm is fixed at the end portion of its outer-surface dividing face to the inside of the casing, and the inner-surface position of the lower diaphragm rises in relative fashion with respect to the position of the casing. In the end, the diaphragm (fixed region) moves upward with respect to the rotor (rotating region) and this upward movement becomes another cause of rotor-fin contact.
For steam turbine operation in recent years, reduction in startup time is being demanded and it has traditionally been becoming common to increase the temperature and flow rate of steam within a shorter time. Particularly in this case, differences in temperature are prone to occur in a radial direction of the diaphragm, hence making thermal deformation more likely to result.
The thermal deformation moves the casing (fixed region) upward relative to the rotor (rotating region) and becomes causative in rotor-fin contact.
In order to address the problems concerned, JP-2002-228013-A, for example, discloses a technique relating to a sealing unit having a labyrinth sealing device which includes sealing fins between a rotating region such as a rotor, and a fixed region such as a diaphragm, and which uses a highly workable, metallic free-cutting member (abradable material) at a position opposed to each fin. According to JP-2002-228013-A, the technique disclosed therein is effective, for example, for avoiding damage to the fins and thus reducing a quantitative increase in steam leakage, since the fins cut the abradable material in a case of contact between the fins and the abradable material.